1. Field of the Invention
This invention concerns improved solid fertilizer compositions and in particular solid fertilizer compositions useful for preparing aqueous fertilizer solutions for precision plant nutrition.
2. Prior Art
In greenhouses, nurseries and other intensive horticulture environments, best results are attained when macro- and micronutrients are carefully delivered to the growing plants. Many growers choose to utilize compound high analysis water-soluble fertilizers. Typically, these fertilizers are marketed as solids which are dissolved to prepare concentrated stock solutions which are then diluted into irrigation water by means of proportioners or injection devices.
The Grace Sierra Horticultural Products Company markets a wide range of water-soluble solid compound fertilizer formulations under the Peters Professional.RTM. trademark. These formulations are designed to dissolve quickly and completely with no precipitation. It is also desired that a fertilizer formulation have good long-term stability as a stock solution so as not to form precipitates which can clog proportioners and irrigation lines. This has led to a limitation with water-soluble fertilizer formulation available heretofore. Calcium is an important nutrient which is called for in many plant nutrition formulations, but calcium salts cannot be used together with the conventional phosphorous sources--monoammonium phosphate and diammonium phosphate--as the pHs which these phosphate sources provide are too high (not acidic enough) to give adequate solubility to the calcium ions and thus give rise to precipitation of the calcium salts in the stock solution and in the proportioners.
Potassium phosphate or sodium phosphate can be used as soluble phosphorous sources but these are either expensive or not conducive to plant growth. Phosphoric acid can be used but is a liquid and hazardous to handle. Therefore, a grower wishing to fertilize with both calcium and phosphorous, without resorting to the use of an alkali metal phosphate or liquid phosphorous acid, will need to inject these two elements separately.
Secondly, and similarly, the use of chelated trace nutrients (micronutrients) has been widely postulated in order to keep these trace nutrients in solution in stock solutions which also contain the monoammonium and diammonium phosphates. If nonchelated micronutrients (such as simple nitrates or sulfates) are employed with the conventional phosphorous sources, the micronutrients tend to precipitate from solution. Again, these chelated micronutrients increase the cost of the fertilizer formulations.
The present invention employs urea phosphate in water-soluble solid fertilizer formulations. Urea phosphate is a known material. The text, Urea and Urea Phosphate Fertilizers by Clifford W. Fowler (Noyes Data Corporation, Park Ridge, N.J., 1976) at page 23, shows a process for preparing this material and suggests that it may be used as a fertilizer. Similarly, U.S. Pat. No. 4,456,463 discloses the use of urea-phosphoric acid reaction products as ammonia-stabilized liquid components of fertilizers. Chemical Abstracts 106(3)101311q shows a liquid component for use in fertilizers which contains a urea phosphate polycondensation product. Chemical Abstracts 102(9)77486v also shows this. Other references of interest include Chemical Abstracts 101(23)209498s which concerns the biological fate of urea/phosphate fertilizers; Chemical Abstracts 101(21)190398k which discusses the use of urea phosphate fertilizers; Chemical Abstracts 88(19)135558a which describes the reaction of that phosphoric acid reacts with urea and with calcium salts; and Chemical Abstracts 86(11)70783g which deals with the development of liquid multicomponent fertilizers based on potassium and ammonium nitrates and diphosphates, urea, water and trace elements.